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Abstract:

The present invention provides an inductively coupled, magnetically
enhanced ion beam source, suitable to be used in conjunction with
probe-forming optics to produce an ion beam without kinetic energy
oscillations induced by the source.

Claims:

1. A method for producing a focused ion beam, comprising the steps of:
applying RF power to an antenna that inductively couples energy to a
plasma to induce ionization of the plasma in proximity of the antenna;
providing impedance matching circuitry to adjust the electrical phase
shift across the antenna to reduce modulation of the plasma potential;
extracting an ionized beam from a region of extraction in proximity to
the antenna; and applying a focusing mechanism to the ion beam extracted
from the region.

2. The method of claim 1, further comprising the step of providing a
magnet in proximity to the region of beam extraction.

3. The method of claim 2, wherein the magnet is fixed annular magnet.

4. The method of claim 2, wherein the magnet produces between 200 and
1000 Gauss.

5. The method of claim 1, wherein the circuitry is adjustable to vary an
amount of power transferred to the plasma.

6. The method of claim 1, wherein the circuitry is adjustable to vary the
voltage phase across the antenna.

7. The method of claim 1, wherein the coil is positioned so that an axis
of the antenna substantially coincides with an axis of propagation of the
extracted beam.

8. The method of claim 1, wherein the antenna is positioned to minimize
modulation of the plasma potential in the region immediately adjacent to
the source exit aperture.

9. A focused ion beam system, comprising: a vessel enclosing a region of
plasma; an antenna excited by an RF electrical source to induce
ionization of the plasma; circuitry that couples the antenna to the
electrical source to substantially reduce RF oscillations in the ionized
plasma. an extraction mechanism to extract the ionized plasma into a
beam; and a focusing mechanism to focus the beam.

10. The system of claim 9, further comprising a means to dissipate heat
generated in the plasma.

11. The system of claim 9, wherein the antenna comprises a single or
multi-turn helical coil oriented to induce high ionization density in the
plasma in a region in proximity to the coil.

12. The system of claim 9, wherein the focusing mechanism focuses the
beam for milling and deposition.

13. The system of claim 9 wherein a high ion density in the plasma
produces a high beam current without substantial modulation of the plasma
potential and hence axial energy spread of extracted ions.

14. A focused ion beam system for milling and deposition, comprising: a
plasma tube comprising an ionizable, non-metallic plasma gas and a source
aperture at an end of the tube; a helical antenna positioned around the
plasma tube; circuitry in a network comprising the antenna; an extractor
that enables extraction of the plasma from the source aperture into a
beam; and a focusing mechanism to focus the beam.

15. The system of claim 14, further comprising a magnet placed in
proximity of the source aperture.

16. The system of claim 14, wherein the focusing mechanism causes a beam
of high brightness exceeding 2000 A/cm2/sr with an extracted beam
energy of 10 keV.

17. The system of claim 14, wherein the system exhibits an energy spread
that is less than 3 eV.

18. The system of claim 14, wherein the system exhibits an energy spread
that is less than 4 eV.

19. The system of claim 14, further comprising a transformer for coupling
the RF source to the plasma.

20. The method of claim 19, wherein a secondary of the transformer is
center-tapped and coupled through a circuit to ground.

Description:

[0001] This application is a Continuation application of U.S. patent
application Ser. No. 12/704,123, filed Feb. 11, 2010, which is a
Continuation application of U.S. patent application Ser. No. 11/825,136,
filed Jul. 2, 2007, and issued as U.S. Pat. No. 7,670,455, which is a
Continuation of U.S. patent application Ser. No. 10/988,745, filed Nov.
13, 2004 and issued as U.S. Pat. No. 7,241,361, which claims priority
from U.S. Provisional Pat. App. 60/546,142, filed Feb. 20, 2004, all of
which are hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates to the field of focused ion beam
systems.

BACKGROUND OF THE INVENTION

[0003] Focused ion beam (FIB) systems are widely used in characterization
or treatment of materials on microscopic to nanoscopic scales. For
example, focused ion beam systems are used in manufacturing operations
because of their ability to image, mill, deposit and analyze with great
precision. Ion columns in FIB systems using gallium liquid metal ion
sources (LMIS), for example, can provide five to seven nanometers of
lateral resolution. Such focused ion beam systems are commercially
available, for example, from FEI Company, Hillsboro, Oreg., the assignee
of the present application. Although use of liquid metal ion sources has
increased, their application is often limited due to metal ion
contamination and relatively low obtainable beam currents. Lower beam
currents result in lower erosion rates and hence longer processing times
in production applications and in laboratories.

[0004] In contrast to FIB systems are broad ion beam systems suitable for
semiconductor processing on a relatively large scale. For example, a
broad beam system may be used for semiconductor doping over nearly the
entire surface of a silicon substrate wafer. RF sources have been used
for large-area wafer processing. For such uses no probe forming optics
are employed.

[0005] A challenge exists in generating high current, low energy beams for
implantation from an RF plasma source. A complication associated with
using an RF driven ion source for low energy ion implantation is the
undesirable oscillations imparted to the plasma potential, through
capacitive coupling from the antenna to the plasma. The plasma potential
can have peak-to-peak oscillations of several hundred volts, thereby
dramatically modulating the extracted beam energy. Such a highly
modulated beam is unsuitable for low energy ion implantation, due to the
broadened projected ion range into the target.

[0006] Ion energy modulation is even less suitable for FIB systems, due to
the associated chromatic aberrations generated in the beam. The
relationship between the energy spread due to RF modulation of the beam
and the chromatic blur of the beam is given by:

d c = Δ E E 0 C c α i ( 1 )
##EQU00001##

where, dc is the diameter of the chromatic disc, Cc is the
chromatic aberration coefficient for the optical system and αi
is the convergence half-angle of the beam as it forms the focused spot at
the target. Eo is the energy to which the ions are accelerated by
the extraction optics. The term ΔE is the energy spread that
results from modulations in the plasma potential due to capacitive
coupling from the antenna, coupled with the fundamental axial energy
spread of ions from the source that is determined by the potential
gradient in the pre-sheath region of the plasma. Clearly, the modulations
imparted by the RF source substantially and undesirably impact the focus
of the beam. At least partially for this reason, it is believed that RF
sources have not been successfully used with FIB systems.

[0007] For precision milling and deposition, one desires high beam current
for faster production times, focused into a small spot. Hence, high
source brightness and minimal optical aberrations are required. The
"brightness" of a beam from a plasma source is proportional to the beam
current density from the source and inversely proportional to the mean
thermal ion energy for ions existing in the plasma. This is expressed by
the equation (2):

β max = J i E 0 π E ∥ ( 2 )
##EQU00002##

Here βmax is the beam brightness assuming zero aberrations
introduced from the extraction optics, Ji is the current density
extracted from the plasma, Eo is the energy to which the ions are
accelerated by the extraction optics and E.sub.∥ is the mean thermal
ion energy. Clearly, beam brightness increases when the current density
is increased and the mean thermal ion energy is decreased.

[0008] The current density, Ji, is given by:

Ji=0.6niq {square root over (kBTe Mi)} (3)

where, (ni) is the plasma ion density, (Te) is the mean
electron energy within the plasma, (q) is the fundamental unit of charge,
(kB) is Boltzmann's constant, and (Mi) is the mass of the of
the ion in the plasma. Clearly, current density is increased by
increasing the plasma ion density and increasing the mean electron energy
in the plasma. Hence, for the values indicated above, the optimum
αi is determined to be ˜7.5 mrads, resulting in an image
side brightness of ˜7×103 A cm-2 sr-1 and a
source brightness of ˜1.5×104 A cm-2 sr-1 at
20 keV.

[0009] To summarize, we want high current density and low thermal energy
to obtain high beam brightness. To achieve high current density, we want
high plasma ion density and high mean electron energy. What is needed,
therefore, is a high-density focused plasma ion beam system with low
thermal ion energies to facilitate high brightness.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method for forming a plasma that
is suitable as a high brightness ion source. The plasma is inductively
coupled to a compensated RF antenna and can be used in conjunction with
focusing optics to produce a focused ion beam for milling and deposition.
According to an aspect of the present invention, the RF antenna can be
implemented as a helical coil that surrounds a plasma tube. An RF current
source is applied to the antenna to induce ionization of the plasma gas
in the tube. An impedance matching circuit is provided to allow efficient
power transfer to the plasma with appropriate phase shift across the
antenna to eliminate plasma potential modulation. The ionized plasma is
extracted into an ion beam and focused by ion optics. The ion beam so
formed is substantially free of undesirable energy oscillations arising
from the RF antenna. Because the RF source imparts only small or ideally
no oscillations to the plasma potential, the consequent axial energy
spread of the beam arising there from is small. Hence, the ionizing
source does not cause substantial chromatic aberration. Moreover, the RF
source imparts to the plasma a high ion density. When coupled with
focusing mechanisms, the high-density beam is highly suitable for milling
and deposition.

[0011] The foregoing has rather broadly outlined features and technical
advantages of the present invention in order that the detailed
description of the invention that follows may be better understood.
Additional features and advantages of the invention will be described
hereinafter. It should be appreciated by those skilled in the art that
the conception and specific embodiment disclosed herein may be readily
utilized as a basis for modifying or designing other structures for
carrying out many useful purposes of the present invention. It should
also be realized by those skilled in the art that such equivalent
constructions do not depart from the spirit and scope of the invention as
set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a more complete understanding of the present invention, and the
advantages thereof, reference is now made to the following descriptions
taken in conjunction with the accompanying drawings, in which:

[0013] FIG. 1 shows a preferred embodiment of an ion plasma source
according to the method of the present invention.

[0014] FIG. 2 shows a circuit for adjustment of power transfer to the
plasma.

[0018] FIG. 1 shows a diagram of an embodiment of an ion plasma source 10
according to the method of the present invention. A coil 1000 is
capacitively coupled by impedance 210 to an RF source 200. Note that the
capacitances shown in FIG. 1 are nominal values that readily can be
selected by one of skill in the art according to the frequency of
operation of the coil, as will be described further below. Coil 1000 is
preferably a multi-turn coil that wraps around a dielectric plasma tube
2000 so that the axis of the coil substantially coincides with the axis
of chamber 2000 and the beam axis.

[0019] When driven by source 200, coil 1000 forms a helical RF antenna.
Driving the coil with an RF source can impart a time-varying potential to
the plasma, due to capacitive coupling. That is, the coil can produce a
radial electric field that modulates the plasma. This is undesirable
because it creates a spread in the beam energy, resulting in chromatic
aberration. However, in a preferred embodiment of the present invention,
the antenna is driven at one end by a signal that is out of phase with
the signal at the opposite end by as much as 180°. This creates a
region interior to the coil where the potential fluctuations are
substantially zero at all times. In this region there is substantially no
time-varying modulation of the plasma arising from the time-varying
voltage across the coil 1000. Thus, the phase of the antenna can be
adjusted to minimize modulation of the ionization potential of the plasma
in the region where ions are extracted in response to an applied
acceleration field. The energy of the ions extracted from the plasma,
according to this method, is substantially un-modulated by the RF voltage
across the antenna.

[0020] However, source 200 does indeed cause electrons to move. Because of
the orientation of the coil, free electrons in the plasma circulate
around the plasma skin, causing them to collide with atoms to produce
ions. This can produce plasma of very high ion density with relatively
low thermal ion energy. A fixed-strength annular magnet about 5 to 10
milli-meters thick, or a variable-strength electromagnet 3000, that
produces an axial field strength of nominally 200 to 1000 Gauss is placed
between an end of the coil and a region 3500 of extraction, and is
provided to increase plasma density. The magnet reduces electron
diffusion and loss to the walls of the plasma chamber. Thus, the RF
source is inductively coupled to the plasma and the annular magnet
increases the plasma density in the extraction region.

[0021] A split Faraday shield 6000 can be used to screen out the
capacitive field of the coil, but this is less desirable for two main
reasons. First, a degree of capacitive coupling is required to ignite the
plasma. Using a split Faraday shield usually requires another external
power source (e.g., a Tesla coil) to ignite the plasma. Second, split
Faraday shields typically result in some energy loss, due to Eddy
currents induced in the shield. Without the Faraday shield, the balanced
antenna approach still results in a sufficient time varying electric
field in areas of the plasma chamber to cause the initial field
ionization required to initiate the plasma.

[0022] A beam voltage 400 is electrically connected to a beam energy cap
420, which has an additional low pass filter 410 to ensure negligible RF
pick-up to the beam voltage. An extractor voltage source 600, that is
negative with respect to the potential applied to the source electrode
4000, is applied to the extraction electrode 4500. Skimmer electrode 5000
is at ground potential and provides an aperture through which the dense
ion beam passes to produce an ion beam that can be focused with
appropriate optics. Thus, the beam is extracted from the extraction
region 3500, with a beam waist formed in the skimmer aperture 5000, and
thus propagates along the beam axis in response to an applied
acceleration. Alternatively, beam voltage 400 can be electrically
connected directly to the source electrode 4000 instead of to the beam
energy cap 420.

[0023] Note that the configuration of electrodes in the extraction region
shown in FIG. 1 is representative, but other electrode configurations
could be implemented instead. Indeed, the invention contemplates that
extraction optics generating a collimated or divergent beam will be
optimum, as opposed to forming a waist in the beam. Essentially,
extraction optics that result in the least emittance growth and minimal
Coulomb interactions are the most favorable for a high brightness ion
source.

[0024] FIG. 2 shows a circuit of the present invention, including a plasma
impedance, Zp, 2010 in parallel with an unknown coil inductance
characteristic 1010. In series with the parallel combination of the
plasma impedance 2010 and inductance 1010 is capacitance 8000. This
series-parallel combination is in parallel with a second capacitance
7000. This parallel-series-parallel combination is in series with a third
capacitance 210. This entire network is in parallel with the source 200.
Clearly, the phase shift across the coil and plasma impedance can be
controlled by the selection of capacitance values 210, 7000 and 8000. One
can therefore select capacitance values 210, 7000, and 8000 to obtain a
phase shift across the coil and plasma of 180 degrees.

[0025] Suppose for example that the source is at 13.56 Mega-Hertz (MHz)
with a 50 ohm output impedance. Then choosing the following capacitances
results in a phase shift across the coil and plasma of about 180 degrees:
capacitance 210=50 pico-Farads (pF), capacitance 7000=330 pF, capacitance
8000=340 pF. The combined components--capacitances 210, 7000, and 8000,
and inductance 1010--may be viewed as an impedance matching network that
matches the 50 ohm source impedance to the plasma impedance 2010 load.
When the network is matched, maximum power from the source is transferred
to the plasma impedance load. These values were implemented for an Argon
plasma tube with an inner diameter of 20 milli-meters (mm), an outer
diameter of 26 mm, and a length of 100 mm. The wall of the plasma tube
was 3 mm-thick quartz. The coil was 30 mm long with three turns and a
diameter of about 50 mm.

[0026] Thus, the present invention provides a circuit adjustment to
achieve a maximum transfer of power to the plasma, coupled with
negligible modulation of the plasma potential, resulting in negligible
axial energy spread of the extracted ions. FIG. 3 shows a graph of the
axial ion energy distribution resulting from the antenna in a balanced
state and an unbalanced state. When the antenna is in the balanced state,
that is, when the phase shift across the antenna is about 180 degrees,
then the axial ion energy is very narrowly distributed about zero
electron-Volts (eV). This is shown by the solid curve in FIG. 3, which
exhibits a full width at half magnitude of less than 3 eV. In contrast,
when the antenna is unbalanced, the axial ion energy is distributed over
a broad energy range. This is shown by the dashed curve in FIG. 3, which
exhibits a full width at half maximum of greater than 70 eV.

[0027] The embodiment described above minimizes the effects of capacitive
coupling on the ions, leaving only the influence of the pre-sheath
potential gradient. The potential gradient of the pre-sheath region is
finite, but small, and is generally about half the mean electron energy
(Te), where Te is only 3 eV for the type of source described
above, giving an inescapable lower limit to the axial energy spread
(ΔE) of ˜1.5 eV.

[0028] The present invention may be conveniently operated at low RF power,
nominally imparting 25 Watts to the plasma. At this power level a
brightness of ˜200 Acm-2sr-1
(Amperes/centimeter-squared/steradian) can be generated at only 5 keV
with an ion current density of 19.6 mA cm-2. This implies a thermal
energy of ≦0.15 eV and a plasma density of
˜8×1011 cm-3. Pulse plasma densities of
1×1014 ions cm-3 have been attained with this source,
implying that a source brightness of >1×105 A cm-2
sr-1 is obtainable at a beam energy of 50 keV, with current density
of: Ji=0.6niq {square root over (kBTe
Mi)}˜2.4 Acm-2, where E0=50 keV, and
E.sub.∥=0.15 eV. This yields

β max = J i E 0 π E ∥ > 1
× 10 5 A cm - 2 sr - 1 . ##EQU00003##

[0029] FIG. 4 shows a focused ion beam system 100 that includes an
evacuated envelope 10 upon which is located a plasma source 11 with an RF
antenna, implemented as described above, to provide a dense plasma for
ion beam focusing column 16. Ion beam 18 passes from source 11 through
column optics 16 and between electrostatic deflection mechanism 20 toward
specimen 22, which comprises, for example, a semiconductor device
positioned on movable X-Y stage 24 within lower chamber 26. A
turbo-molecular pump 8 is employed for evacuating the source and
maintaining high vacuum in the upper column optics region. The vacuum
system provides within chamber 26 a vacuum of typically between
approximately 1×10-7 Torr and 5×10-4 Torr, with nominally 10
mTorr in the plasma source and <1×10-6 Torr in the column optics
chamber.

[0030] High voltage power 34 is connected to ion source 11 as well as to
appropriate electrodes in focusing column 16 for forming an approximately
0.1 keV to 50 keV ion beam 18 and directing the same downward. RF power
supply 33 and impedance matching circuit 27 is also provided to energize
a coil of source 11, as described above. Deflection controller and
amplifier 36, operated in accordance with a prescribed pattern provided
by pattern generator 38, is coupled to deflection plates 20 whereby beam
18 may be controlled to trace out a corresponding pattern on the upper
surface of specimen 22. In some systems, the deflection plates are placed
before the final lens, as is well known in the art.

[0031] The ion beam source 11 is brought to a focus at specimen 22 for
either modifying the surface 22 by ion milling, material deposition, or
for the purpose of imaging the surface 22. A charged particle multiplier
40 used for detecting secondary ion or electron emission for imaging is
connected to video circuit and amplifier 42, the latter supplying drive
for video monitor 44 also receiving deflection signals from controller
36. The location of charged particle multiplier 40 within chamber 26 can
vary in different embodiments. For example, a preferred charged particle
multiplier 40 can be coaxial with the ion beam and include a hole for
allowing the ion beam to pass. A scanning electron microscope 41, along
with its power supply and controls 45, are optionally provided with the
FIB system 8.

[0032] Signals applied to deflection controller and amplifier 36, cause
the focused ion beam to move within a target area to be imaged or milled
according to a pattern controlled by pattern generator 38. Emissions from
each sample point are collected by charged particle multiplier 40 to
create an image that is displayed on video monitor 44 by way of video
circuit 42. An operator viewing the image may adjust the voltages applied
to various optical elements in column 16 to focus the beam and adjust the
beam for various aberrations.

[0033] Focusing optics in column 16 may comprise mechanisms known in the
art for focusing or methods to be developed in the future. For example,
two cylindrically symmetric electrostatic lenses can be implemented to
produce a demagnified image of the round virtual source. Because of the
low axial energy spread in the extracted beam, chromatic blur is minimal
and efficient focusing of the beam can be achieved even at low
acceleration voltages (ie low beam energies). These properties in
conjunction with appropriate focusing optics can be used to generate
nanometer, to micrometer scale spot sizes with a range of kinetic
energies (0.1 keV-50 keV) and beam currents from a few pico-amperes to
several micro-amperes.

[0034] A large transfer of power from the coil to the plasma is desirable
to achieve high ionization density. Hence, efficient power dissipation is
required to limit the operating temperature of the plasma source.
Desirably, heat is conducted away from the plasma tube efficiently. This
can be achieved by forming a dielectric and metallic shell around the
plasma, inside the coil, with high thermal conductivity to efficiently
conduct heat away from the plasma.

[0035] The source can be expected to have a predictable and relatively
long life span (ie >>1000 hours). This type of source differs from
a DC plasma source, because ions leave the plasma and strike the
surrounding walls at energies that are nominally 15-20 eV (5.2 Te for an
argon plasma), resulting in negligible sputtering of source chamber
material. A typical DC source has a plasma potential that is nominally
50-500 V higher than a cathodic electrode, resulting in significant
sputtering of the cathode that ultimately results in the end of life for
these sources. Substantial elimination of plasma potential modulation,
according to the methods described herein, also substantially reduces the
probability of ions striking the source electrode with energy above the
sputter threshold. Ions transit the plasma sheath in a time that is
substantially less than the period of the RF signal. Hence, ions leave
the plasma with kinetic energies that are determined largely by the
temporal plasma potential induced by the capacitive field from the
antenna.

[0036] The realization of very high plasma densities (up to
1014/cm3), low thermal ion energies (down to 0.1 eV), low axial
energy spread (1.5-3 eV), the ability to operate with either inert or
reactive gases, and the potential for very long life due to minimal
erosion of source materials, makes a magnetically enhanced, inductively
coupled plasma source ideal to be used in conjunction with probe forming
FIB optics.

[0037] The present invention can provide beam currents from a few
pico-amperes to current greater than 10-11, greater than 10-10
amps, greater than 10-9 amps, greater than 10-8 amps, greater
than 10-7 or current of several micro-amperes. A source brightness
of at least 104 A/cm2/sr, at least 105 A/cm2/sr, and
up to 106 A/cm2/sr or more at 50 keV can be achieved. The axial
energy spread is less than 3 eV, less than 2.5 and could be as low as 1.5
eV. This contrasts sharply with present day Liquid Metal Ion Sources
(LMIS), which can provide a beam brightness on the order of 106
A/cm2/sr, but with an energy spread on the order of 5 eV. Also, LMI
sources are generally only suitable for generation of beam currents in
the pico- to nano-ampere range. A further advantage of the present
invention is the ability to operate with any inert gas as well as many
reactive gases, (e.g., O2, N2, SF6, etc. . . . ). The ion
beam from the present invention is capable of being focused into a beam
diameter of a few nanometers, up to several tens of micrometers. Inert
gas beams can readily be generated making the invention suitable for
applications where gallium or other metallic ion beams might be
problematic.

[0038] In fact, the axial energy spread is dictated only by the static
potential gradient of the pre-sheath region of the plasma. Ions can be
generated at any point on the pre-sheath potential gradient, with ions
created at the top of the gradient ultimately acquiring more kinetic
energy than those created at the bottom. The energy distribution is
determined by the height of the potential hill, which is determined by
the mean electron energy in the plasma (Te), according to:

ΔV˜kBTe 2q (4)

With a mean electron temperature of 3.48×104 K (3 eV), the
resulting potential drop in the pre-sheath is -1.5V.

[0039] As a point of reference, a 100 nm diameter, 100 pA argon FIB, with
a nominal landing energy of 20 keV requires an image side brightness (B)
of ˜7200 A cm-2 sr-1 according to:

B = 4 I π 2 α i 2 d 2 ( 5 )
##EQU00004##

with αi being the convergence angle, d the spot size, and I
the beam current. If an axial energy spread (ΔE) of nominally 2 eV
is assumed, along with a demagnifying two lens optical system having a
chromatic aberration coefficient, Cc=86 mm and a spherical
aberration coefficient, Cs=120 mm, the beam is chromatically
dominated under the optimum conditions. Furthermore, we can assume that
our beam will have equal contributions from chromatic blur and the
demagnified geometric source size. Hence, the contributions from
chromatic aberration disk (dc) and the geometric spot size will each
be ˜100 {square root over (2)}=71 nm. Hence, for the values
indicated above, the optimum αi is determined to be ˜7.5
mrads, resulting in an image side brightness of ˜7×103 A
cm2 sr-1 and a source brightness of ˜1.5×104 A
cm-2 sr-1 at 20 keV.

[0040] In order to generate a source brightness of 1.5×104
Acm-2 sr-1 in an argon plasma, a magnetically enhanced
Inductively Couple Plasma (ICP) source can be used. In order for this
plasma source to provide a brightness of greater than 1.5×104
A cm-2 sr-1, at a nominal energy of 20 keV, a current density
from the source of ˜225 mA/cm2 is required assuming a thermal
energy of 0.1 eV, according to:

β max = J i E 0 π E ∥ ( 6 )
##EQU00005##

Finally, in order to achieve Ji=225 mA/cm2 we require a plasma
density of at least 9×1012 ions/cm3, from:

n i = J i 0.6 q k B T e • M i
( 7 ) ##EQU00006##

where ni=plasma ion density (m-3), Ji=2250 A m-2,
Te=3.48×10-23 J K-1, q=1.6×10-19 C,
Mi=39.948×1.66×10-27 kg. The plasma source
invention described herein provides all the necessary plasma attributes
(ion density, mean thermal ion energy and axial energy spread) to result
in a high brightness ion source suitable for nanometer scale FIB
applications. Thus, the present invention provides a low energy spread
(<3 eV) and low mean thermal ion energy (<0.15 eV) at low RF power,
on the order of 25 Watts imparted to the plasma. Also, the source
exhibits very high beam current stability (<0.1% drift per hour). At
higher RF powers the beam brightness increases while still maintaining
low thermal ion energies within the plasma to enable realization of a
targeted brightness with a plasma density below that achieved in a pulsed
mode.

[0041] An alternative embodiment of the present invention is shown
schematically in FIG. 5. An RF generator 950 is presented with a source
resistance 951. This is coupled to shunt capacitance 952. A primary
winding 955 wrapped around a ferrite toroidal core 957 with a Faraday
shield 956 is coupled to the RF source through inductance 954 and
capacitance 953. Capacitance 953 compensates for leakage inductance 954.
Ferrite core 957 may be formed from two commercially available
cemented-together cores wrapped with Teflon tape, each core rated at 1
kilo-Watt (kW). In a preferred embodiment, the primary winding is
distributed around the toroid as copper tape. Wrapped over this is more
Teflon insulation tape, followed by a Faraday shield, followed by another
layer of Teflon.

[0042] There are two balanced secondary windings 958 and 959 that may be
implemented as single passes of copper tape traversing through the
interior of core 957. These may be held in place by the hardware of the
capacitors and stand-off feed-through connectors for the antenna wires.
The secondary windings are coupled together through adjustable
capacitance 960 and are coupled to ground through the parallel
combination of like capacitances 961 and 962 and like resistances 963 and
964. The opposite ends of the windings 958 and 959 are coupled across the
parallel combination of the antenna inductance 965 and the plasma
impedance 966. Values for the circuit components may be obtained from a
SPICE program assuming a value for the plasma impedance.

[0043] The transformer formed by the primary and secondary windings
provides an impedance transformation of one to (T1/T2)**2, where T1 is
the number of turns in the primary and T2 is the number of turns in the
secondary. This aids in transforming the low plasma impedance 966 to the
50 ohm impedance of the generator. For example, with a turns ratio of 7
to 2, the impedance transformation is one to 12.25.

[0044] The secondary is center-tapped and coupled to ground to provide a
balanced circuit independent of the impedance match. The secondary
windings and antenna, in conjunction with capacitances 960, 961, and 962,
form a series resonant circuit. Variable capacitance 960 enables tuning
of the circuit that is relatively insensitive to changes in plasma
impedance. The balun leakage inductance 954 is compensated by capacitance
953, and the parallel capacitance 952 completes the match to 50 ohms. The
matching provided by this embodiment is relatively insensitive to
component tolerances. Moreover, the balanced circuit produces an RF field
that is substantially symmetrical across the plasma length.

[0045] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein without departing from
the spirit and scope of the invention as defined by the appended claims.
Because the invention can be used in different applications for different
purposes, not every embodiment falling within the scope of the attached
claims will achieve every objective. Moreover, the scope of the present
application is not intended to be limited to the particular embodiments
of the process, machine, manufacture, composition of matter, means,
methods and steps described in the specification. As one of ordinary
skill in the art will readily appreciate from the disclosure of the
present invention, processes, machines, manufacture, compositions of
matter, means, methods, or steps, presently existing or later to be
developed that perform substantially the same function or achieve
substantially the same result as the corresponding embodiments described
herein may be utilized according to the present invention. Accordingly,
the appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means, methods,
or steps.